Glare is one of the troublesome factors when driving a vehicle. Many efforts have been made to solve the glaring problem. One of the most effective ways is to provide an electrochromic unit for the rearview mirror of the vehicle. The electrochromic unit deepens the color and thus reduces the reflection rate of the mirror accord to the degree of the glare, thereby minimizing the glaring effect. FIG. 1 is a schematic block diagram showing a conventional electrochromic rearview mirror system. The rearview mirror system includes an interior rearview mirror assembly 1 inside the vehicle, two exterior rearview mirror assemblies 2 and 3 by two sides of the vehicle, respectively, and a control device 4. Each of the rearview mirror assemblies 1, 2 and 3 includes an electrochromic unit EU1, EU2, EU3 which performs color change of the mirror under the control of the control device 4.
Further referring to FIG. 2, the control device 4 includes a forward light sensor S1, a rear light sensor S2, a voltage source 141, a micro-controller 142 and a drive circuit 143. The sensors S1 and S2 are disposed at the forward and rearward sides of the interior rearview mirror assembly 1 for detecting the intensities of the forward light and rear light, respectively. First of all, the micro-controller 142 determines whether it is daytime or nighttime according to the forward light intensity detected by the sensor S1. For example, when the intensity of the forward light is detected to be greater than 50 Lux, it is determined to be daytime, and the micro-controller 142 disables the drive circuit 143. On the contrary, if the forward light is no greater than 50 Lux, the micro-controller 142 further determines whether a glaring situation occurs in response to the light intensity difference detected by the sensors S1 and S2, and controls the drive circuit 143 to apply a suitable voltage to all of the three electrochromic units EU1, EU2 and EU3 to change the color of the mirrors to a relatively low reflection rate when a glaring situation occurs.
The drive circuit 143 receives a voltage from the voltage source 141 which is the ignition or the vehicle battery voltage. The voltage is reduced to a level suitable for the electrochromic purpose, e.g. a voltage ranged between 0 and 3.5V, and provided for the electrochromic units EU1, EU2 and EU3 under the control of the micro-controller 142. The higher the voltage level is applied to the electrochromic unit, the deeper the color of the mirror is, and the lower the reflection rate of the mirror becomes. The reflection rate is lowered from an uncolored initial level, e.g. 55% in general.
The conventional electrochromic rearview mirror system as mentioned above, in spite of solving some glaring problems, cannot solve the problems thoroughly. For instance, in addition to the headlight of an oncoming vehicle, sunrise or sunset sunlight is another important glaring source. The rear sunlight results in powerful stimulation for eyes. The above-mentioned electrochromic mechanism does not work in daytime, so the glaring effect resulting from sunlight cannot be avoided.
On the other hand, when a glaring situation occurs, the three electrochromic units are activated simultaneously. In other words, when the sensor S2 at the interior rearview mirror detects a high intensity headlight from an oncoming vehicle right behind the user's vehicle, all the three mirrors change colors. Meanwhile, if the user would like to change to a right lane or a left lane, the colored exterior mirrors will have adversely effect on the observation of the other lanes. On the contrary, when a vehicle with a high intensity headlight is approaching the user's vehicle at the right or left side, it is possible for the sensor S2 to detect no glare so that all the three mirrors remains highly reflective. However, in fact, the right or left rearview mirror has been bothered by the glare.
Furthermore, the uncolored initial reflection rate 55% is generally suitable for the daytime viewing, but a little insufficient for some people with weaker sight in the dark. When the mirrors are darkened by the glare, the night-vision problem is even more serious.
The use of the forward light sensor S1 to detect the ambient light for determining whether it is the daytime or nighttime may result in an erroneous result. Due to the “forward” arrangement, as shown in FIG. 3, the light sensor S1 may detect in the nighttime the headlamp light of a coming car in opposite direction as the ambient light, and thus wrongly determine that it is the daytime. As a result, the anti-glare function would be improperly disabled.